1. Field of the Invention
The present invention relates to a target signal detecting apparatus for discriminating and detecting a target signal from a noise signal whose amplitude characteristic exhibits the Weibull distribution.
2. Description of the Prior Art
In recent years, it has been observed that various kinds of noise included in signals such as the noise included in a signal received by a radar due to the reflection of the transmitted signal from the surface of the sea and the noise included in a signal received by an ultrasonic detecting apparatus are distributed in accordance with the Weibull distribution as described for example in U.S. Pat. Nos. 4,242,682, 4,318,101 and 4,513,286, and in an article entitled "Suppression of WeibullDistributed Clusters Using a Cell-Averaging LOG/CFAR Receiver", co-authored by the inventor herein.
Assuming now that the amplitude intensity x of a signal X follows the Weibull distribution, the probability density function Pc(x) of the Weibull distribution is given as follows: ##EQU3## where b is a constant determined by the nature of the signal and C a distributed parameter. This probability density function Pc(x) exhibits any one of various distribution forms depending on the value of the distributed parameter C. For example, it exhibits the exponential distribution when the value of the distributed parameter C is 1, the Rayleigh distribution when the value is 2 and the Gaussian distribution when the value is 2.5.
Where the amplitude of a target signal is smaller than the amplitude of a noise signal, the detection of the target signal is impossible by a conventional discrimination method in which a suitable amplitude threshold is established to discriminate the two signals from each other.